feat(init-generator): add ability to specify a package manager of choice

[jamesgeorge007]

What kind of change does this PR introduce?
Feature

Did you add tests for your changes?
Update test suite

If relevant, did you update the documentation?
N/A

Summary
#2768

Does this PR introduce a breaking change?
No

Other information
N/A

[alexander-akait]

Let's fix CI

[evenstensberg]

lgtm

[codecov]

Codecov Report

Merging #2769 (b4ef065) into master (59d644d) will decrease coverage by 0.05%.
The diff coverage is 90.00%.

@@            Coverage Diff             @@
##           master    #2769      +/-   ##
==========================================
- Coverage   94.10%   94.05%   -0.06%     
==========================================
  Files          31       32       +1     
  Lines        1680     1698      +18     
  Branches      490      490              
==========================================
+ Hits         1581     1597      +16     
- Misses         99      101       +2     

Impacted Files	Coverage Δ
.../webpack-cli/lib/utils/get-available-installers.js	86.66% <86.66%> (ø)
packages/generators/src/init-generator.ts	98.11% <100.00%> (+0.03%)	⬆️
packages/webpack-cli/lib/utils/index.js	100.00% <100.00%> (ø)

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